Water is essential for plant survival, growth and reproduction. Assimilation of carbon dioxide by photosynthesis is directly linked to water loss through the stomata. Crop productivity which is closely linked to biomass production is dependent on plant water use efficiency (WUE) especially in water limited conditions (Passioura 1994 and Sinclair 1994, in Physiology and Determination of Crop Yield). Water use efficiency over a period of plant's growth can be calculated as the ratio of biomass produced per unit of water transpired (Sinclair 1994). Instantaneous measurements of water use efficiency can also be obtained as the ratio of carbon dioxide assimilation to transpiration using gas exchange measurements (Farquhar and Sharkey 1994, in Physiology and Determination of Crop Yield). Since there is a close correlation between crop productivity and water use efficiency, many attempts have been made to study and understand this relationship and the genetic components involved. To maximize the productivity and yield of a crop, efforts have been made to try to improve the water use efficiency of plants (Condon et al., 2002, Araus et al., 2002, Davies et al., 2002). Higher water use efficiency can be achieved either by increasing the biomass production and carbon dioxide assimilation or by reducing the transpiration water loss. Reduced transpiration, especially under non-limiting water conditions can be associated with reduced growth rate and therefore reduced crop productivity. This poses a dilemma on how to improve crop productivity and yield under water limited conditions but also maintain it under irrigated or non-limited water conditions (Condon et al., 2002).
Improvements to water use efficiency, to date, have used plant breeding methods whereby high water use efficiency varieties were crossed with the more productive but lower water use efficiency varieties in hope of improvements in crop yield under water limited conditions (Condon et al., 2002, Araus et al., 2002). Quantitative trait loci (QTL) approaches to identifying the components of water use efficiency have been the most common methods historically used (Mian et al., 1996, Martin et al., 1989, Thumma et al., 2001, Price et al., 2002), and more recently attempts have been made to engineer improved plants by molecular genetic means.
The first gene associated with water use efficiency was ERECTA. The ERECTA gene was first identified as a gene functioning in inflorescence development and organ morphogenesis (Torii et al., 1996),). It was later found by QTL mapping to be a major contributor to transpiration efficiency, defined as water transpired per carbon dioxide assimilated, an opposite indicator to water use efficiency in Arabidopsis (Masle et al., 2005). ERECTA encodes a putative leucin-rich repeat receptor-like kinase (LRR-RLK). The regulatory mechanism of LRR-RLK is yet to be understood although it was suggested due to, at least in part, the effects on stomatal density, epidermal cell expansion, mesophyll cell proliferation and cell-cell contact. The normal transpiration efficiency was restored upon complementation using wild type ERECTA in mutant exacta. However, it is not known whether overexpression of ERECTA in transgenic Arabidopsis will result in reduced transpiration efficiency or enhanced water use efficiency. It is the only report showing a plant receptor-like kinase to be involved in transpiration efficiency or water use efficiency.
Another Arabidopsis gene implicated in water use efficiency is the HARDY gene, found through the phenotypic screening of an activation tagged mutant collection (Karaba et al., 2007). Overexpression of HARDY in rice resulted in improved water use efficiency by enhancing photosynthetic assimilation and reducing transpiration. The transgenic rice with increased expression of HARDY exhibited increased shoot biomass under optimal water conditions and increased root biomass under water limited conditions. Overexpression of HARDY in Arabidopsis resulted in thicker leaves with more mesophyll cells and in rice increased leaf biomass and bundle sheet cells. These modifications contributed to enhanced photosynthetic activity and efficiency (Karaba et al., 2007).
Protein kinases are a large family of enzymes that modify proteins by addition of phosphate groups (phosphorylation). Protein kinases constitute about 2% of all eukaryotic genes, many of which mediate the response of eukaryotic cells to external stimuli. All single subunit protein kinases contain a common catalytic domain near the carboxyl terminus while the amino terminus plays a regulatory role.
Plant receptor-like kinases are serine/threonine protein kinases with a predicted signal peptide at the amino terminus, a single transmembrane region and a cytoplasmic kinase domain. There are more than 610 RLKs potentially encoded in Arabidopsis (Shiu and Bleecker 2001). Receptor-like kinases are often part of a signaling cascade. They interpret extracellular signals, through ligand binding, and phosphorylate targets in a signaling cascade which in turn affect downstream cell processes, such as gene expression (Hardie 1999).
Identification of genes that can be manipulated to provide beneficial characteristics is highly desirable. So too are means and methods of utilizing the identified genes to effect the desirable characteristics. The receptor-like kinase identified as At2g25220 in the TAIR database is one serine/threonine kinase, and a member of the large gene family of receptor-like kinases with over 600 members in Arabidopsis (Shiu et al., 2001). However, except for annotation of the sequence as a kinase no function or role for the At2g25220 gene has been disclosed. In the present invention a high water use efficiency gene (HWE) has been identified that when its expression or activity is inhibited results in beneficial phenotypes, such as, enhancement of plant biomass accumulation relative to the water used. This occurs under both water limited and non-limited conditions and ensures better growth and therefore greater productivity of the plants.